Overpruning Weakens and Threatens Palms

The Sarasota Herald Tribune was kind enough to print my guest column advising people that simply put: you cannot improve an established palm's health by sawing off fronds. I wrote the column in response to a letter to the editor commenting on a previous article about Texas Phoenix Palm Decline, a disease that is killing our native cabbage palms, among other species. The first link above takes you to the article as it ran on September 7th. The version below is my original submittal:

I am writing to comment on the unclear recommendations of an arborist in Venice who wrote in response to your article documenting the threat posed by Texas Phoenix Palm Decline to both exotic palms and our native cabbage palm.

I am a student of cabbage palms, so I wasn’t surprised to read your article regarding the threat posed by Texas Phoenix Palm Decline (TPPD). I’ve been commuting to USF St. Pete and driving through Palmetto where, ironically, some of the most extensive cabbage palm mortality is evident.

But I had secretly hoped something might happen to limit the spread of this disease that could decimate cabbage palms in both urban landscapes and natural areas such as Myakka River State Park. Unfortunately, today I learned the disease has also been found in Duval County. It is rapidly becoming a statewide threat.  A November 18^th workshop on this threat is being planned by Hillsborough County Extension office.

Our state tree is known as a tough, resilient tree – simultaneously the most flood, wind, and fire resistant native upland tree in the state. Incredibly, it can withstand the death of all its roots. When transplanted virtually all the roots die and the palm must grow all new roots. Because a rootless tree cannot support many leaves, re-located cabbage palms are heavily pruned, down to just a few leaves. They eventually recover, although takes many months.

The arborist who wrote in advocated putting palm trees “on a regular maintenance cycle” that, he argued, would make them less susceptible to damage. If he was referring to an appropriate fertilization schedule, I would agree. But unfortunately many regular palm pruning techniques actually put palms at risk.

Palms are commonly overpruned, and cabbage palms are frequently the most abused – subjected to something perversely called “the hurricane cut.” Research after the 2004 hurricanes concluded that cabbage palms are the second-most wind tolerant tree in Florida. So there is no need to prune cabbage palms in anticipation of a windstorm. In fact, heavy pruning actually reduces the ability of cabbage palms to weather storms. But people still routinely overprune, sometimes removing more than 4/5 of the palm’s living leaves. This debilitating setback not only vastly reduces the photosynthetic capacity of the plant, but actually invites insect attack.

When fronds that aren’t completely dead are removed, the wounds release chemicals known as kairomones, -- chemical signals from one species that benefits another species, but that does not help the organism that released the chemical. In this case the wounded palm sends an aggregation signal to Palmetto Weevils.

The Palmetto Weevil is a native insect, our largest weevil, and it has been joined here in Florida by an exotic, invasive palm weevil, the Silky Cane Weevil. These two sometimes team up to cause more problems for palms. It is believed that feeding by the Silky Cane Weevil releases kairomones that attract the Palmetto Weevil, which then kills the tree, at least in Canary Island Palms. Could this one-two punch also affect cabbage palms? It is definitely possible and could be aggravated by the large number of field-grown cabbage palms that are wounded (pruned) and moved around the state. All that wounding releases kairomones and could lead to escalating palm weevil numbers.

Not only can removing green or yellow fronds invite insect attack and weaken the tree, but it even complicates diagnosis of palm problems. Palm experts can interpret the lowermost leaves to discern nutritional deficiencies and other problems. And since one of the first symptoms of TPPD is greater number of dying leaves, aggressive pruning can mask the disease. We don’t know if overpruned trees are more susceptible to attack by the leafhoppers believed to carry TPPD, but common sense argues that unnecessarily stressing plants is not a path to optimum health and disease resistance.

Bottom line: Everyone has their own aesthetics, but you cannot improve an established palm’s health by sawing off fronds. It’s fine to remove dead, brown fronds, but make sure that whoever prunes your palms knows that taking any green or yellow fronds weakens your trees, invites insect attack, and could possibly contribute to a reduction in the numbers of our state tree, the cabbage palm.